Mujoco grip parameters

Discussion in 'Simulation' started by Giusebar, Jun 13, 2018 at 8:53 AM.

  1. Hello, I'm attempting to grasp a box with a UR10 and a custom gripper. I am able to grasp the box with a pinch grasp but the block slides out of the finger. Based on this discussion I have tried to set up the values of solimp and solref as suggested and that seems to help.

    Anyway I'm a bit confused by the meaning of those parameters in relation to actual physical contact between the gripper and the box.

    I wonder whether there are some example on how to set up parameters that allows to simulate properly the different contact behaviour depending on the type of materials (E.g. rubber on cardboard).

    Or what is the rule of thumbs to set up those parameters?

    Thanks,
    Giuseppe
     
  2. Emo Todorov

    Emo Todorov Administrator Staff Member

    Think of the *deformation* as behaving like a mass-spring-damper. solimp sets the mass, in a way that may depend on position (i.e. the mass of the deformation increases with constraint violation), while solref sets the stiffness and damping. There is internal scaling applied, so that the same solref/imp parameters result in similar behaviors for very different masses of the rigid bodies. The details are in the documentation here

    http://www.mujoco.org/book/computation.html#soParameters

    In general, the best way to set parameters for a given physical system is to do system identification: obtain measurements form the system, and optimize the model parameters so as to make the model fit the data. Normally people do this to infer inertias and frictions. But since MuJoCo has a rich model of deformation dynamics, these parameters also have to be identified.

    If you still have problems with slip in grasping, and can also try the noslip solver.